Axinite-(Mn)

Chemical formula: Ca₄Mn₂+₂Al₄[B₂Si₈O₃₀](OH)₂

Axinite-(Mn) is a rare, manganese-rich member of the axinite group, valued for its characteristic wedge-shaped crystals with colors ranging from yellow to brown and strong pleochroism.

## Characteristics Axinite-(Mn), formerly known as manganoaxinite, is a calcium, manganese, and aluminum borosilicate belonging to the axinite group. It forms flattened, sharp-edged crystals with a characteristic wedge shape, often aggregated into fan-like or rosette-like clusters. Crystal surfaces can be striated. It is a brittle mineral, and its most distinct optical feature is strong pleochroism, meaning a change in color depending on the observation direction. ## Physical Properties It is characterized by a hardness of 6.5-7 on the Mohs scale and a vitreous luster. It is a transparent to translucent mineral. Its density is approximately 3.33 g/cm³. It also exhibits piezoelectric and pyroelectric properties, meaning it generates an electric charge under pressure or temperature change. ## Colors and Varieties Typical colors of axinite-(Mn) include shades from clove-brown, through reddish-brown, orange, to honey-yellow. Bluish or violet specimens are rarer. The color change observed at different angles (pleochroism) is very distinct, showing shades of violet, yellow, and brown. ## History and Name The mineral's name refers to its chemical composition – it is a member of the axinite group in which manganese (Mn) is the dominant divalent element. The group name "axinite" comes from the Greek word "axine" (axe), referring to the typical flattened and sharp shape of the crystals. It was first described in 1909. ## Uses Due to its rarity and attractive appearance, axinite-(Mn) is primarily a prized collector's stone. The purest and largest crystals are occasionally faceted for collectors of rare gemstones, but its brittleness and perfect cleavage make processing difficult.

Properties

Mohs hardness: 6.5-7
Luster: Vitreous
Streak: White
Density: 3.33
Cleavage: Good on {100}
Fracture: Uneven to conchoidal
Transparency: Transparent to translucent
Crystal system: Triclinic

Diagnostic features

## Identification A key diagnostic feature of axinite-(Mn) is its unique, sharp, wedge-shaped crystal habit. Strong pleochroism, visible even to the naked eye in transparent crystals as a distinct color change when rotating the specimen, is also very helpful in identification. A vitreous luster and striations on crystal faces are also characteristic. ## Distinguishing from Similar Minerals It can be confused with other minerals from the axinite group, such as axinite-(Fe) or axinite-(Mg). Differentiation requires advanced chemical analysis (EDS, WDS) to determine the dominant element. It is distinguished from titanite by the absence of titanite's characteristic adamantine luster and the typical axinite crystal shape. Epidote usually forms more elongated, prismatic crystals with a greenish color. ## Crystal Forms Crystals are tabular, flattened, and have a very characteristic, sharp-edged, axe-like or wedge-shaped appearance. They often form fan-like, rosette-like, or radial aggregates where individual crystals intergrow.

Geological environment

## Genesis Axinite-(Mn) is a mineral typical of low-grade metamorphic rocks, especially skarns and hornfelses formed at the contact of granitoid intrusions with manganese-rich carbonate rocks. It also forms as a result of contact metasomatism and in alpine fissures and hydrothermal veins. ## Mineral Associations It co-occurs with minerals such as quartz, calcite, rhodochrosite, rhodonite, bustamite, tephroite, garnets (spessartine), epidote, clinozoisite, prehnite, and vesuvianite. ## Localities The most famous and prized specimens come from the Wessels and N'Chwaning mines in the Kuruman area of South Africa. Other important localities include Franklin and Sterling Hill in New Jersey (USA), where it occurs in zinc-manganese deposits. It is also found in Broken Hill, Australia, in Sweden (Långban), Italy (Val d'Aosta), and Japan.

Rarity

Rare

For collectors

## Quality Criteria The most highly valued specimens are those with well-formed, sharp, undamaged crystals of high transparency and intense, attractive color (e.g., orange, honey-yellow). Composition is also very important – aesthetic rosette or fan-like aggregates on a contrasting matrix are more desirable than single, isolated crystals. Specimens from classic localities, such as mines in the Kalahari Basin, fetch higher prices. ## Popular Localities The best specimens in the world are considered to come from mines in the Kalahari manganese field in South Africa, especially from the N'Chwaning mine. They are characterized by exceptional color, transparency, and form. Historically important specimens from Franklin, USA, are also notable, though they appear less frequently on the market.

Care and storage

## Cleaning Specimens should only be cleaned with lukewarm water and a mild soap, using a soft brush. After washing, they should be thoroughly rinsed with clean water (preferably distilled) and left to air dry completely or gently dried with a soft cloth. ## What to Avoid Ultrasonic and steam cleaners should be avoided, as they can cause fractures along cleavage planes. The mineral is sensitive to sudden temperature changes and strong acids, which can damage it. Prolonged exposure to intense sunlight may lead to fading in some specimens. ## Storage Due to its brittleness and relatively good cleavage, axinite-(Mn) specimens should be stored separately, in padded boxes or on stands, to prevent scratches and impacts. It should not be stored with harder minerals such as quartz or corundum.

Sources

IMA Mineral List (CNMNC)